Fix a -fpermissive build issue.

[orbea]

On gentoo with a recent ffmpeg git the build fails. (https://github.com/FFmpeg/FFmpeg/commit/5541cffa17a8c45004e5ceeda52d4d6b2acee037)

samu: job failed: /usr/lib/ccache/bin/c++ -D_FILE_OFFSET_BITS=64 -D__STDC_CONSTANT_MACROS -I. -fPIC -std=c++11 -MD -MT CMakeFiles/libffmpegthumbnailerobj.dir/libffmpegthumbnailer/moviedecoder.cpp.o -MF CMakeFiles/libffmpegthumbnailerobj.dir/libffmpegthumbnailer/moviedecoder.cpp.o.d -o CMakeFiles/libffmpegthumbnailerobj.dir/libffmpegthumbnailer/moviedecoder.cpp.o -c ../libffmpegthumbnailer/moviedecoder.cpp
../libffmpegthumbnailer/moviedecoder.cpp: In member function ‘void ffmpegthumbnailer::MovieDecoder::initializeVideo(bool)’:
../libffmpegthumbnailer/moviedecoder.cpp:198:41: error: invalid conversion from ‘const AVCodec*’ to ‘AVCodec*’ [-fpermissive]
  198 |     m_pVideoCodec = avcodec_find_decoder(m_pVideoStream->codecpar->codec_id);
      |                     ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                         |
      |                                         const AVCodec*
samu: subcommand failed

I am not sure this is the best way of solving the issue so please confirm, thanks!

[orbea]

@dirkvdb When you have a chance can you please review this PR? :)

[dirkvdb]

Hi, I cannot approve a const_cast as a solution.

I can't understand how avcodec_find_decoder would return a const* in your case but unfortunately I currently have no time to investigate this.

[dirkvdb]

Fixed by pull request #201